There have been several primers and processes developed over the years to improve the adhesion of materials to polypropylene and other polyolefins. Polyolefins are generally inert, paraffin-like polymeric substrates which are non-polar and very difficult to paint, coat or bond. They are further characterized as having low critical surface tensions of wetting, e.g., 35 dynes/cm or less which make them difficult to wet with paints, coatings and adhesives. Some of the processes developed to date to enhance the adhesion to polyolefins are flame treatment, corona discharge treatment, photo sensitized ultraviolet irradiation, gas plasma treatment, acid etching, electron beam irradiation and perhaps others. These processes generally enhance adhesion by either increasing wetting via oxidation (increase the critical surface tension of wetting), by cross-linking to increase surface mechanical strength, by increase in surface porosity to increase mechanical bonding by diffusion, by general surface cleaning or by combinations of these effects.
Primes have been developed for increasing adhesion to polyolefins and are generally non-polar or polar/non-polar in nature to help provide a "bridge" between the substrate (the polyolefin) and the adhesive, coating or paint. Paints, coatings and adhesives are not necessarily always polar, but generally are, and the polar/non-polar primer has both an affinity for the non-polar substrate and the polar paint, coating or adhesive.
The patents of Bragole U.S. Pat. Nos. 3,600,289; 3,607,536; 3,619,246; 3,627,609; 3,892,885; 4,321,307; and 4,764,370 teach various manifestations of photosensitized irradiation techniques to enhance bonding to polyolefin substrates. Such manifestations included the use of isocyanates or silanes or magnesium reacted phenolic resins with photosensitized ultra violet irradiation. Other manifestations included the presence of isocyanate at the time of photosensitized ultraviolet irradiation while others used photosensitized irradiation in combination with reactive products such as those containing isocyanates, acrylic monomers or epoxies. Some of the current techniques, while effective for some applications, produce surface changes which are not permanent. Others suffer from the fact that large quantities of hazardous waste have to be removed which is very expensive. Others suffer from universality and some are too expensive to have had widespread use.
The primers appear to be more effective with paints and coating than with adhesives. While the primer adheres well to the polyolefin it frequently suffers from inadequate adhesion to the adhesive.
I have developed a method for modifying such primers to enhance the adhesion between the primers and adhesives. The invention broadly comprises a method for bonding coatings to a polyolefin and to a polyolefin primer composition which primer has significantly enhanced bonding characteristics to the coating and preferably an adhesive coating. As used in this disclosure, polyolefin includes polyethylene, polypropylene, polybutylene, polyisoprenes and their copolymers. The primers are preferably halogenated polyolefins and are treated to form free radical species which species chemically bond to the coating, preferably an adhesive.
With this method, commercially available primers for polyolefins, such as Eastman's E343-3, E343-1, can be used. These primers inherently have excellent adhesion to polyolefin surfaces but only fair adhesion to the coatings applied to the primer. The surfaces of the primers are modified, such as by ultraviolet irradiation, for greater interactions with paints and other coatings but adhesives in particular. While ultraviolet irradiation has been used to modify polyolefin surfaces and more commonly has been used to cure coatings, generally containing unsaturated acrylic monomers, it is not believed to have been used to alter polyolefin primers. The alteration of the primer in a unique way enhances the universality of the primer and particularly enhances its adhesion to adhesives. The treatment of the primer with ultraviolet light does not necessarily cure the primer since the primer does not contain unsaturated acrylics. The treatment of the primer is believed to yield a variety of free radical species which increase the adhesion of the primer dramatically to adhesive systems.